April 23rd 2024

How to Control Fuel Switching between Electric Heat Pumps and Fossil Fuel Boilers or Use CO2 refrigerant Heat Pumps for low ambient building and DWH heat

How to Control Fuel Switching between Electric Heat Pumps and Fossil Fuel Boilers or Use CO2 refrigerant Heat Pumps for low ambient building and DWH heat

During the recent webinar, "Using IESVE for Loads, Sizing and Heat Pump Modeling to Achieve Decarbonization" I was asked an intriguing question by an engineer who was attempting to switch fuels for heating equipment at specific outdoor conditions, i.e. to avoid an energy-inefficient operation:

"Is there a way to model an AWHP that cuts off at say 0°F (-18°C) and use a Fossil Fuel boiler for below 0°F?"

My Response:

Yes, it is possible to sequencing the Heat Pumps to turn off at a specific condition and engage supplemental boiler (or other heating device).  We can achieve this with a 5-step workflow and model an Auxiliary Heating device on the Hot Water Loop (HWL), as shown in Figure 1.

Figure 1 – Hot Water Loop (HWL) in IESVE 2023 showing 3 air-source CPHP (with ability to provide both heating and cooling) with a single gas-fired condensing boiler in an auxiliary configuration

Step 1: Create a Hot Water Loop with Heat Pump(s)

In the “Heating equipment set” tab of the HLW dialogue shown in Figure 2, setup heating devices that are considered to be the “primary” heating devices for this hot water loop; e.g. Central Plant Heat Pumps (CPHP).

Figure 2 – view of “Heating equipment set” tab.  This is where the CPHP devices are sequenced and their size is shown in ApacheHVAC.

Step 2: Setup the Heat Pump Controls

In the Heat Pump Equipment dialog, ensure the Operating Control Mode is set to ‘Load Priority’. The CPHP’s in Figure 3 have been set to provide both Heating and Cooling. This approach will also successfully work for CPHP’s which are set to Heating Only. The operating mode control is set to Load Priority.  This approach will also work operating control mode set to Heating Priority.

Figure 3 – Air-source Heat Pump Settings via CPHP dialog

Step 3: Set the Minimum source temperature for the Heat Pump

In the CPHP dialogue, set the CPHP “Minimum source temperature for heating mode operation” to the desired temperature.  In this example, the CPHP will only operate anytime the ambient temperature is above 10°F (-12°C) and the space requires heat.

When the temperature is below 10°F (-12°C), the auxiliary device will engage to provide heat. 

Step 4: Setup the downstream heating equipment

Setup the downstream heating equipment via the “Auxiliary heat source” tab of the Hot water loop dialogue. Note the “Percentage of reference load for sizing” has been set to 100% in Figure 4.  This will size the auxiliary heating device to be able to address 100% of the HWL load as it assumes the air-source CPHP’s will be unavailable below 10°F (-12°C). 


Figure 4 – Auxiliary heat source 

This is the setup, if the auxiliary device is intended to supplement the CPHP devices due to a reduction in heating load from cold temperatures, then the percentage of reference load should value in this field should be reflective of the difference between the heat generated by the CPHP at coldest temperatures, minus the total hot water loop load from the sizing calculations performed by the engineer.

Step 5: Review Results

In Figure 5, plotting the Heat delivered by CPHP (blue) and auxiliary boiler (red) for a week in January in Duluth, MN using VistaPro in IESVE, it can be seen that the Heat Pumps are 100% off below when the external dry-bulb temperature (green) is below 10°F (-12°C).  Simultaneously, this is the same trigger for the auxiliary boiler to turn on, to provide 100% of the heat required whenever the temperature is below 10°F (-12°C). 

Figure 5 – Simulated Results

This example is one of many potential ways to use the auxiliary heat source in conjunction with the Heat Pumps in the ‘Heating equipment set’.

It is possible to use an air source heat pump with CO2 refrigerant at super low ambient temperatures instead of a gas-fired condensing boiler as an auxiliary heat source.  The challenge with this approach is finding the performance data on CO2 heat pumps.  

Click here to learn more about IESVE and how it can be used for Loads Sizing and Heat Pump Modeling, and look out for more FAQs coming soon to DiscoverIES.